In known ignition systems of the above-described type, as, for example, disclosed in DE-OS 25 49 586, when the voltage across the resistor reaches a predetermined voltage indicative of sufficient current flow through the primary winding for energy storage in the ignition coil, this voltage causes another transistor in the circuit to become conductive, causing part of the base current from the power transistor to be shunted. This prevents further current increase in the primary winding.
It is a disadvantage of this type of current control that high losses occur in the power output transistor during the time that its conductivity is decreased. This results in the requirement for a relatively expensive transistor having a high power rating. However, even with use of such a transistor the destruction of the power output transistor and therefore a malfunction of the whole ignition system cannot always be prevented.
In another known system, described in DE-OS 2 406 018, the current through the primary winding is interrupted completely when the voltage across the resistor reaches the above-mentioned predetermined voltage. At the same time, a circuit connected in parallel to the primary winding is switched to the conductive state allowing a discharge current to flow. The primary circuit remains in the cutoff state and the parallel circuit in the conductive state either for a predetermined time interval or until such time as the discharge current becomes less than a predetermined minimum value. This system has a disadvantage that two power transistors may have to be provided in the circuit of the primary winding of the ignition coil and also that special control circuits must be provided for an electric switch in the parallel circuit.